There have been broadly employed radiographic images such as X-ray images for diagnosis of the conditions of patients on the wards. Specifically, radiographic images using an intensifying-screen/film system have achieved enhancement of speed and image quality over its long history and are still used on the scene of medical treatment as an imaging system having high reliability and superior cost performance in combination. However, these image data are so-called analog image data, in which free image processing or instantaneous image transfer cannot be realized.
Recently, there appeared digital system radiographic image detection apparatuses, as typified by a computed radiography (also denoted simply as CR) and a flat panel detector (also denoted simply as FPD). In these apparatuses, digital radiographic images are obtained directly and can be displayed on an image display apparatus such as a cathode tube or liquid crystal panels, which renders it unnecessary to form images on photographic film. Accordingly, digital system radiographic image detection apparatuses have resulted in reduced necessities of image formation by a silver salt photographic system and leading to drastic improvement in convenience for diagnosis in hospitals or medical clinics.
The computed radiography (CR) as one of the digital technologies for radiographic imaging has been accepted mainly at medical sites. However, image sharpness is insufficient and spatial resolution is also insufficient, which have not yet reached the image quality level of the conventional screen/film system. Further, there appeared, as a digital X-ray imaging technology, an X-ray flat panel detector (FPD) using a thin film transistor (TFT), as described in, for example, the article “Amorphous Semiconductor Usher in Digital X-ray Imaging” described in Physics Today, November, 1997, page 24 and also in the article “Development of a High Resolution, Active Matrix, Flat-Panel Imager with Enhanced Fill Factor” described in SPIE, vol. 32, page 2 (1997).
To convert radiation to visible light is employed a scintillator panel made of an X-ray phosphor which is emissive for radiation. The use of a scintillator panel exhibiting enhanced emission efficiency is necessary for enhancement of the SN ratio in radiography at a relatively low dose. Generally, the emission efficiency of a scintillator panel depends of the phosphor layer thickness and X-ray absorbance of the phosphor. A thicker phosphor layer causes more scattering of emission within the phosphor layer, leading to deteriorated sharpness. Accordingly, necessary sharpness for desired image quality level necessarily determines the layer thickness.
Specifically, cesium iodide (CsI) exhibits a relatively high conversion rate of X-rays to visible light. Further, a columnar crystal structure of the phosphor can readily be formed through vapor deposition and its light guide effect inhibits scattering of emitted light within the crystal, enabling an increase of the phosphor layer thickness (as described in patent document 1).
However, the use of cesium iodide (CsI) alone results in reduced emission efficiency. For example, there was disclosed a technique for use as an X-ray phosphor in which a mixture of cesium iodide (CsI) and sodium iodide (NaI) at any mixing ratio was deposited on a support (substrate) to form sodium-activated cesium iodide (CsI:Na); recently, there was also disclosed a technique in which a mixture of cesium iodide (CsI) and thallium iodide (TlI) at any mixing ratio was deposited on a substrate to form thallium-activated cesium iodide (CsI:Tl), which was further subjected to a heat treatment at a temperature of 200 to 500° C. to achieve enhanced visible-conversion efficiency to employ as an X-ray phosphor (as described in patent document 2).
However, such an activator was different in crystal structure from cesium iodide, so that its increased content resulted in disorder of a columnar crystal structure, producing problems such as deterioration of sharpness.
There was further disclosed a panel in which a sublayer comprising a phosphor parent compound (CsBr) and exhibiting a lower density than a phosphor layer was provided, resulting in superior adhesiveness between a phosphor and a support and leading to enhanced sensitivity (as described in patent document 3). However, it was proved that such a technique was insufficient in sensitivity and inferior in moisture resistance.